Carbon dioxide is a major contributor to global warming, which is one of the most serious environmental problems facing society. Carbon dioxide is believed to have the greatest adverse impact on the observed greenhouse effect causing approximately 55% of global warming. Relying on fossil fuels as the main source of energy in many parts of the world has contributed to the rise of carbon dioxide emissions to unprecedented levels. Many industries, for instance natural gas sweetening, hydrogen production for ammonia and ethylene oxide, oil refining, iron and steel production, desalination, energy production, and cement and limestone manufacturing, represent major sources of carbon dioxide emissions.
Carbon capture and storage (CCS) is an option to reduce carbon dioxide emissions. CCS is based on the separation and capture of carbon dioxide produced by fossil fuel power plants and other sources either before or after combustion. A number of CO2 capture technologies have been used such as oxy-fuel combustion, pre-combustion decarbonization, post-combustion processing and chemical looping combustion. Among the post-combustion capture techniques, the most promising and most effective are solvent absorption, adsorption using solid sorbents, membrane separation, and cryogenic fractionation technology.
Key parameters for selecting an effective solvent for CO2 absorption include high absorption, fast reaction kinetics, low degradation rate, and low regeneration energy as well as the ability to handle large amounts of exhaust streams.
U.S. Pat. No. 8,540,954 proposes the use of molten salts as an absorption medium, wherein the absorption medium comprises molten salts containing at least one halide of an alkali or alkali earth metal that has a content of dissolved metal oxide, which reacts with the carbon dioxide and creates a metal carbonate. The molten salts, which contain a metal carbonate, are heated at temperatures of 600 to 1600° C. to release the metal oxide and carbon dioxide. However, the main disadvantage of applying chemical absorption process is the thermal energy requirement for separating the CO2 from the solvent.
EP2529825 describes the use of carbonate looping technology where flue gas is made in contact with solid material to capture and store CO2, which can then be released by decarbonation at elevated temperatures.
WO 2012/120173 describes the capture of CO2 in a tube exchanger with amino-alcohol-impregnated alumina supports under combined conditions of TSA, PSA, vapour entrainment and subsequent reconditioning of the sorbent.
JP2012091130 describes a CO2 recovery device which can recover CO2 from exhaust gas by using an amine liquid with high efficiency.
U.S. Pat. No. 8,647,412 describes the use of a sorbent material derived from an amino-functionalized alkoxysilane and a polyamine, wherein the sorbent material is present in an amount equal to or greater than 10 g/l, wherein at least some of the sorbent material resides in the porous channel walls and forms CO2 adsorption sites within the interior of the porous channel walls. However, amine-based sorbents are known to require costly feed materials and need significant amounts of solvents through the preparation processes (Fuel 108 (2013) 112-130).
US20110005390 describes the use of solid particles made of a cross-bounded, highly porous polymer substrate and CO2 absorbing functional nucleophilic groups grafted on the particle surface. Other methods of making these structures for CO2 capture are described in US20070149398 as a high surface area structure that includes a plurality of pores in the high surface area structure. The CO2 sorption structure is an inorganic/organic hybrid structure that is about 10 to 70% organic and about 30 to 90% inorganic.
A device and method for capturing CO2 from fluid flow is described in U.S. Pat. No. 8,211,394. It includes a flow-through apparatus and a CO2-absorbing filter treated with an alkaline material which is housed within the flow-through apparatus. The flow-through apparatus receives fluid flow and the CO2 is absorbed by the CO2-absorbing filter. The absorbed CO2 is then converted into CaCO3 which is combined with volcanic ash to form a useful cement material. US20100218507 describes a system for removing CO2 from the environment using four major steps: capture, separation, transformation, and sequestration.
The Solvay process has been considered for the capture of CO2 and the production of useful and reusable carbonate products, as well as the desalination of saline water (Desalination 251 (2010) 70-74). Solvay is a process for the manufacture of sodium carbonate (soda ash), where ammonia and carbon dioxide are passed through a saturated sodium chloride solution to form soluble ammonium chloride and a precipitate of sodium bicarbonate according to Reaction (1) below. The sodium bicarbonate is heated to form the washing soda and the ammonium chloride solution is reacted with calcium hydroxide to recover the ammonia according to Reactions (2) and (3), respectively.NaCl+NH3+CO2+H2O→NaHCO3+NH4Cl  (1)2NaHCO3→Na2CO3+CO2+H2O  (2)2NH4Cl+Ca(OH)2→CaCl2+2NH3+2H2O  (3)
Many methods have applied the Solvay approach. WO 2007/139392 describes a combined process for removing carbon dioxide from combustion gases and desalination of water by reaction of carbon dioxide of the input gas stream with an alkaline solution based on ammonia and saline water. A similar process is described in U.S. Pat. No. 7,309,440, which involves the desalination of seawater and separation of CO2 from a gas turbine exhaust; seawater is mixed with NH4OH and released via a series of nozzles in several vertical levels in a process unit.
EP1961479 describes a process where CO2 is contacted with concentrated brine and ammonia. Such an approach is also described in U.S. Pat. No. 8,486,182 where ammonia is mixed with seawater to produce ammonia-saturated seawater which is then contacted with an exhaust gas so that carbon dioxide in the exhaust gas is absorbed in the ammonia-saturated seawater.
Another method for combining the desalination of seawater and the removal of CO2 is described in WO 2001/096243, where seawater is mixed with ammonia and then pumped into a chamber and dispersed at many points near the top as a fine spray, exposing the salt to the CO2 gas. WO 2010/057261 describes a process for producing soda ash from brine waste. The process involves reacting brine waste with carbon dioxide and ammonia to produce soda ash, wherein at least a portion of the ammonia is regenerated from ammonium chloride produced during the reaction. The regeneration is achieved through the use of a weak base anion exchange resin. US 2012/0298522 also describes a system and method for soda ash production, but by integrating the Solvay process with an electrochemical process to produce a less CO2-intensive Solvay process and an environmentally friendly sodium carbonate product. Desalination methods that include carbonate compound precipitation are described in U.S. Pat. No. 7,931,809 where both feed water and waste brine are subjected to carbonate compound precipitation conditions and carbon dioxide sequestration.
One of the major drawbacks of the Solvay process as used in the above mentioned documents is the presence of ammonia, which is considered an environmental and health hazard. At room temperature, ammonia is a colourless, highly irritating gas with a pungent, suffocating odour. It is highly corrosive and hydroscopic. Although ammonia gas is not flammable outside its explosion limits (16 to 25%), containers of ammonia may explode when exposed to high temperatures. Exposure to high concentrations of ammonia can cause severe injuries such as burning of the skin, nose, throat and respiratory tract, which can cause bronchiolar and alveolar oedema, and airway destruction leading to respiratory distress or failure. Ammonia is not involved in the overall Solvay reaction, but it plays a key role in buffering the solution at a basic pH; without ammonia, the acidic nature of the water solution will hinder the precipitation of sodium bicarbonate (Desalination 251 (2010) 70-74).
It is therefore desirable to find a process for desalinating water and capturing CO2 which does not require the use of ammonia. It is also desirable to develop a process which does not make use of energy intensive steps such as electrolysis.